Conventionally, in papermaking processes, when or after a filler-containing stuff is diluted to a final concentration for feeding into a papermaking machine, a retention aid is added thereto so that the pulp and filler are prevented from flowing from the papermaking machine to white water, thereby improving retention.
As the retention aid, water-soluble polymers such as water-soluble high-molecular-weight polyethyleneoxide and cationic polyacrylamide are usually used.
However, the retention aids containing these water soluble polymers must be used in a relatively large amount in order to improve the retention. As a result, they are problematic in that huge flocs are produced, thereby considerably deteriorating paper formation.
In order to solve this problem, a so-called dual system in which a cationic polymer is used in combination with an anionic compound or polymer is recently noticed. Typical examples thereof include a method in which an anionic inorganic compound such as bentonite is added after addition of a cationic polymer (Patent Document 1), and a method in which an anionic colloidal silica is added after addition of a cationic polymer (Patent Document 2).    Patent Document 1: Japanese Patent Laid-Open (Kokai) No. 04-281095 (claims).    Patent Document 2: Japanese Patent No. 2945761 (claims).